This invention relates to a vehicle lamp control sensor and particularly to such a sensor with a response curve that resembles that of the human eye.
A number of systems have been developed to eliminate the necessity for manually turning on and off vehicle lamps. Such systems typically employ a photocell to detect ambient light levels. The photocell is then coupled to a switch for turning a lamp on or off in response to the level of the ambient light. Examples of such systems include auto headlamp control sensors which automatically turn head lamps on in the dark, and off in daylight.
Such systems generally employ a photocell such as a cadmium sulfide photoconductive cell with a photocell filter to provide an electrical response that varies with changing ambient light levels. In particular, cadmium sulfide photocells change their resistance as the light level changes, and this resistance change is sensed by a level detector in a control circuit to turn the lamp on or off at preset levels.
The use of cadmium sulfide photocells in lamp control sensors presents a number of problems. First, photocells are somewhat sensitive to temperature. This results in an undesirable change in the response of the system with temperature. Further photocells exhibit significant drift in sensitivity over time. Also, due to manufacturing processes, photocells can vary significantly between manufacturing lots. This can result in undesirable variation in the spectral response between otherwise identical photocells.
Moreover, photocells have a spectral response curve that differs significantly from the spectral sensitivity of the human eye. Because of the enhanced response of photocells in the blue end of the visible spectrum, even with suitable filtering, photocell based systems are quite unlike the human eye in their spectral response. As a consequence, a photocell sensor will switch lights on or off at times that may appear inconsistent and inappropriate to the human observer. For example, a typical photocell type sensor may have a response curve that has a maximum response at about 525 nanometers. The human eye has a maximum response in the green region of the visible spectrum at about 550 nanometers under photopic (daylight) conditions. When it is cloudy outside, the clouds cause a shift in the spectral composition toward the blue region. To a human observer, it therefore appears to get darker outside even if the overall light level has not changed, since the observer has less sensitivity in the blue region. This is partially due to the loss of red wavelengths which effects the photocell more than the eye. The photocell, on the other hand, may show an increased response on a cloudy day due to its increased sensitivity in this region. Thus, the photocell perceives it to be less dark than the human because the cloudy sky is rich in the blue spectrum. As a result, for example, the photocell will not turn lights on in cloudy conditions at a time when to a human observer it appears dark enough for the lights to be on.
Thus, it is desirable to have an automatic vehicle lamp sensor which is not sensitive to temperature, and which exhibits a consistent spectral response between manufacturing lots. Further, it is desirable to have a vehicle lamp sensor that has a spectral response curve that more closely matches that of the human eye so that its responses will match perceived changes in light levels.
A lamp sensor achieving the above-mentioned desirable features, is provided according to this invention. The lamp sensor includes a sensor, such as a photodiode, for receiving ambient light and for providing an electrical signal in response to the ambient light. In addition, a filter means is positioned between the sensor and the ambient light for selectively transmitting particular wavelengths of the ambient light to the sensor. The filter has a spectral transmissivity curve such that the combination of the sensor and filter yield a spectral response that approximates that of the human eye. Finally, the sensor is coupled to a means for turning a lamp on and off at predetermined ambient light levels.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.